1. Field of the Invention
This invention generally relates to a linear motion rolling contact guide assembly suitable for use in carrying a relatively large load over a relatively long distance at a relatively high accuracy, and, in particular, to a linear motion guide assembly having a pair of guide rails.
2. Description of the Prior Art
A linear motion rolling contact guide unit or assembly is well known in the art and it is widely used for guiding a moving part in various applications, such as machining tools, robots, and precision measuring, testing and processing apparatuses. Such a linear motion rolling contact guide unit, or LM guide unit for short, generally includes a rail extending straight over a desired length, a slider slidably mounted on the rail and a plurality of rolling members, such as balls and rollers, interposed between the rail and the slider. Typically, a rail has a rectangular cross section with a pair of opposed side surfaces, each formed with a generally V-shaped guide groove, and the slider has a generally U-shaped cross section, including a horizontal section and a pair of vertical sections, each depending from a corresponding side of the horizontal section. Each of the vertical sections of the slider has an inner surface which is formed with a load path section of an endless circulating path, in which a plurality of rolling members are provided. The load path section is located opposite to the corresponding guide groove of the rail so that a channel is defined therebetween. The rolling members are interposed between the guide groove of the rail and the load path section so that a rolling contact is provided between the rail and the slider.
Such a typical linear motion guide unit is useful in many applications; however, its applications are somewhat limited when it is required to carry a load which is relatively large in size as well as in weight over a relatively long distance at high accuracy. That is, if it is required to carry a load which is relatively large in width and weight, the slider must be made larger in width. However, such a structure would bring about a stability problem. In order to cope with such a situation, there has been proposed an improved linear motion guide unit having a pair of rails suitable for carrying a load relatively large in size and weight over a relatively long distance at a relatively high accuracy as shown in FIGS. 4 and 5.
FIG. 4 shows a prior art linear motion guide unit including a pair of guide rails B and C, which are fixedly and parallely mounted on a bed A, a pair of sliders D and E, which are slidably mounted on the guide rails B and C, respectively, and a slide table F which are fixedly mounted on both of the sliders D and E. The slide table F may be fixedly attached to any desired object, which can be moved back and forth relative to the guide rails B and C and thus the bed A linearly as indicated by a double headed arrow G. Each of the guide rails B and C is formed with a pair of guide grooves H and I on both opposite side surfaces thereof extending in parallel to the longitudinal axis of the rail B or C. Each of the sliders D and E has a generally U-shaped cross section and it is mounted on the corresponding guide rail B or C in a straddling manner. And, each of the sliders D and E is formed with a pair of endless circulating paths, each of which includes a load path section, a return path section and a pair of curved connecting path sections, connecting the corresponding ends of the load and return path sections. A plurality of rolling members are provided in each of the endless circulating paths. The load path section is located opposite to the corresponding guide groove H or I of the rail B or C, and the rolling members are partly exposed when located at the load path section and thus brought in contact with the corresponding guide groove H or I. Since the guide grooves H and I are generally V-shaped in cross section, the rolling members located in the load path section carry not only horizontal loads but also vertical loads.
With such a structure, however, when errors are present in the parallel arrangement between the two guide rails B and C, the sliding resistance varies as the slide table F moves along the guide rails B and C so that the accuracy and performance can deteriorate significantly. Besides, its service life can be shortened radically. In addition, since the high accuracy is required in the arrangement between the two guide rails B and C, an extreme care is required in mounting these guide rails B and C on the bed A, which would require high skills and experiences. It is almost impossible to mount these guide rails B and C at an accuracy of a few microns or less if their lengths become a few meters or more.
FIG. 5 shows another prior art linear motion guide unit suitable for carrying a load relatively large in weight as well as in size. This linear motion guide unit includes a pair of guide rails J and K, which are fixedly mounted on a base, a slide table L which is generally U-shaped in cross section and slidably mounted on the pair of guide rails J and K, and a plurality (six in the illustrated example) of endless circulating type bearing units M1-M6 interposed between the rails J and K and the slide table L. The first four bearing units M1-M4 are fixedly attached to the slide table L such that each of them establishes a rolling contact with a corresponding one of top, bottom and a pair of side surfaces of the guide rail J. The remaining two bearing units M5 and M6 are fixedly attached to the slide table L such that they are in rolling contact with the top and bottom surfaces of the guide rail K, respectively. With this structure, vertical loads are carried by bearing units M2, M4, M5 and M6; whereas, horizontal loads are carried by bearing units M1 and M3.
With this structure, since there is no constraints in the horizontal direction between the guide rail K and the slide table L, the relative positional relationship between the guide rail K and the bearing units M5 and M6 may vary as indicated by a double headed arrow N so that errors in the arrangement between the two guide rails J and K can be absorbed. In this case, however, this structure requires the provision of a relatively large number of endless circulating bearing units, which are relatively large in size, in particular its height. In addition, since such an endless circulating bearing unit requires lubrication, a large number of lubricant supplying paths O must be formed in the table L. As a result, such a structure tends to be large in size and complicated in structure, which, in turn, tends to push up the cost. In addition, such a structure prohibits the utilization of readily available, well-known linear motion guide units.